It is well known that two timescales, one based on the Earth's rotation and generically called Universal Time, and the other based on the Earth's orbital motion and generically called Ephemeris Time, diverge. The divergence is explained mostly by a secular slowing down of the Earth's rotation due to tidal braking. This paper investigates the question whether relativistic time dilation plays any role in this divergence, as it has sometimes been proposed. After a detailed analysis from the point of view of general relativity, we demonstrate that relativistic time dilation does not influence the divergence in question. We conclude that there is no reason to refute the present understanding of the secular variation of Earth's rotation.